DESCRIPTION: (provided by the applicant): Previous studies have demonstrated that Lyme disease spirochetes (Borrelia burgdorferi) delivered by ticks are resistant to immune serum unlike cultured bacteria, which are unable to infect mice in the presence of immune serum. The hypothesis being tested in this proposal is that events occurring within feeding ticks transform a homogeneous population of spirochetes into an antigenically and genetically heterogeneous population that is better adapted to evading host immunity and establish an infection in the host. Preliminary studies with the recombinogenic vlsE locus indicate that spirochetes within feeding ticks contain a greater number of vlsE alleles than bacteria within unfed ticks, possibly because tick feeding stimulates recombination at specific loci. The goal of specific aim 1 is to characterize the distribution of vlsE alleles in a spirochete population at distinct states (mice, larval ticks, unfed nymphal ticks, feeding nymphal ticks) as the bacteria go through one complete transmission cycle. The goal of specific aim 2 is to study the mechanism responsible for changing the distribution of vlsE alleles within ticks. An artificial feeding method will be used to introduce clonal populations of bacteria into ticks and these clonal populations will be followed to determine the primary mechanism responsible for generating new alleles during tick feeding. The goal of specific aim 3 is to determine if the tick gut is a major site of bacterial phase variation. Studies with Outer surface proteins (Osp) A and C indicate that spirochetes in the gut turn on and off different lipoproteins during the blood meal. Experiments will be done to characterize the expression pattern of an additional nine well-characterized B. burgdorferi surface proteins to determine the extent of phase variation in the gut. Preliminary data on ospC expression support the hypothesis that OspC is a transmission factor required for the movement of bacteria from the tick gut to the salivary gland. Under specific aim 4, bacterial mutants missing a complete ospC gene will be introduced into ticks to determine if the mutants are capable of crossing the gut epithelium and invading the salivary glands of the tick. Our central hypothesis that tick transmission leads to the introduction of a heterogeneous population of bacteria into the host has important implications for Lyme disease pathogenesis and vaccine development.